Study On Modulation Strategy And Power Loss Analysis Of The New Three-level Converter

In recent years,the new energy industry has been get great developed,three-level converter has been widely used in energy conversion occasion.Based on the Vienna topology,a new topology of three-level converters is proposed in this paper,which could realize the bidirectional energy flow.Taking the new three-level topology as the study subject,implementation of modulation strategy,mid-point potential balance,over-modulation,and power loss are studied.The characteristics of conventional Vienna topology are analyzed in this paper,and the new three-level topology is deduced.The working state of the new topology is studied in detail and its working modes and commutation process are analyzed.Compared with the traditional three-level topology,the characteristics and advantages of the new three-level topology are summarized,and the topology is applied to low and medium voltage inverters.Discontinuous Pulse Width Modulation(DPWM)strategy is studied in this paper.DPWM strategy,which could output voltage and clamp at a specific level within a certain interval in a modulation period,so the switching devices would not work,the efficiency of the converter could improve by reducing the switching loss.According to the different clamping intervals,DPWM strategies can be divided into eight types,and the characteristics of different DPWM strategies are analyzed.Aiming at the complex problem of implementing modulation strategies based on space voltage vector,each DPWM strategy are implemented based on carrier pulse width modulation strategy,which simplifies the modulation algorithm and facilitates the engineering application.the results of analysis indicate that DPWMA has low common mode voltage and good current harmonics,and DPWMA is selected as the modulation strategy of the new three-level converter.Aiming at the problem of neutral-point potential on the DC-side of three-level converter,the cause of the DC-side capacitor voltage unbalancing are analyzed,the influence of different small vectors on the neutral-point potential is studied,and this paper adopts two DPWM strategy hysteresis methods with opposite influence on the neutral-point potential to solve the problem.In order to improve the efficiency of dc voltage utilization,an over-modulation strategy based on carrier is studied.The power loss of the new three-level converter are analyzed,according to study the power loss mechanism of the switching devices,the power loss model of the switching devices is obtained based on the data sheet of the switching device.By analyzing the commutation path of the new three-level converter,the power loss of each switching device in the converter within one modulation period are obtained,and the powe loss calculation models of the new three-level converter under different modulation strategies are established.The power loss of the converter under different conditions are calculated by using the loss calculation model,and it is verified that DPWMA can reduce the power loss effectively of the new three-level converter.The experimental platform of the new topology three-level converter with DSP+FPGA as the controller is established,the validity of the new topology and modulation strategy,the veracity of the power loss model are verified.